An aqueous solution of sodium sulfite is often used in a countercurrent absorption tower to absorb sulfur dioxide from flue gas according to the following reaction:
(1) Na.sub.2 SO.sub.3 +H.sub.2 O+SO.sub.2 .fwdarw.2NaHSO.sub.3 PA1 (2) 2Na.sub.2 SO.sub.3 +O.sub.2 .fwdarw.2Na.sub.2 SO.sub.4 PA1 (3) 2NaHSO.sub.3 +CaCO.sub.3 .fwdarw.CO.sub.2 +Na.sub.2 SO.sub.3 +CaSO.sub.3.1/2H.sub.2 O+1/2H.sub.2 O PA1 (4) 4NaHSO.sub.3 +Na.sub.2 SO.sub.4 +2CaCO.sub.3 .fwdarw.3Na.sub.2 SO.sub.3 +CaSO.sub.3.1/2H.sub.2 O.CaSO.sub.4.1/2H.sub.2 O+CO.sub.2 +H.sub.2 O
If oxygen is also present in the gas stream (such as flue gases) some sulfite is oxidized to sulfate.
The spent absorbent contains an aqueous solution of NaHSO.sub.3, Na.sub.2 SO.sub.4 and Na.sub.2 SO.sub.3.
To regenerate the sodium sulfite the spent absorbent is reacted with calcium carbonate.
The primary reactions, shown below, take place in the regenerators external from the absorber loop so that solids do not enter the absorber.
Existing technology on regeneration of sodium sulfite in the U.S. employe, in the regenerators, lime. However, limestone is far less expensive than lime and reduces the regeneration cost. Japanese processes use limestone to regenerate sodium sulfite but the conditions do not maximize the amount of sodium sulfate coprecipitated by Reaction (4). To remove the sodium sulfate the Japanese FGD (flue gas desulfurization) plants use a costly sulfate removal process that requires sulfuric acid.